High-carbon rimmed steel and method of making it



2,850,373 Patented Sept. 2, 1958 HIGH-CARBON D STEEL AND METHOD OF MG ITVincent C. Boncek, Pittsburgh, Pa., assignor to United States SteelCorporation, a corporation of New Jersey No Drawing. Application May 26,1955 Serial No. 511,438

7 Claims. (Cl. 75-27) This invention relates generally to steelmanufacture and, in particular to rimmed steel with a high carboncontent and a method of making it.

Rimmed steel has peculiar, Well known advantages for certain uses. Ithas been possible heretofore, however, to make rimmed steel having onlya medium or low content of carbon, the maximum carbon contact being .28or 30% (The Making, Shaping and Treating of Steel, 6th ed., p. 573). Incertain applications where rimmed steel is desirable, a higher carboncontent would be advantageous and it is accordingly the object of myinvention to produce such steel and provide a method for making suchsteel. A further object is to produce ingots relatively free of pipewithout the use of hot tops. A still further object is to producehigh-carbon steel having good surface qualities, controlled segregationand freedom from refractory inclusions.

My invention is applicable particularly to the production of steelshaving a carbon content of over .35% and up to 1.10%. Briefly stated,the invention comprises adding to such steel, when it has been teemedinto molds and while it is still liquid, an exothermic rimming agent ofnovel composition. Preferably the rimming agent is added to each moldwhile steel is being teemed thereinto and, specifically, before the moldis half filled. The rimming agent is composed of iron oxide and sodiumfluoride, but includes also an exothermicreaction mixture of granularaluminum and a compound affording a source of oxygen for combinationtherewith, such as sodium nitrate. The amount of the rimming agent usedis from one to four pounds per ton of ingot weight. The following rangesare satisfactory for the several ingredients of the rimming agent:

Table I Percent by weight Iron oxide 40-80 Granular aluminum 2.5-l7Sodium nitrate -34 Sodium fluoride 5-20 The amount of sodium nitrateshould be double the amount of aluminum. The ingredients should be insuch a state of subdivision as to pass through an 8-mesh screen andshould be thoroughly mixed before use.

The iron oxide acts to promote rimming action in the liquid steel byfurnishing oxygen for combination with some of the carbon presenttherein. The sodium nitrate furnishes oxygen for combination with thealuminum. This reaction results in the evolution of a large amount ofheat serving to fuse the iron oxide and sodium fluoride without causinglocalized cooling of the ingot. The sodium fluoride acts as a flux tosweep upward the alumina resulting from the oxidation of the aluminum.The overall result of the addition of the rimming agent, therefore, isto put fused iron oxide and sodium fluoride into the liquid steel as itis being teemed into the mold so that, on standing thereafter, rimmingwill proceed vigorously for such period as necessary to form a case ofthe desired thickness. After the iron oxide of the rimming agent hasreacted, a sodiumaluminate scum remains which acts as a flux andscavenget for any refractory inclusions such as silicates.

As a typical example of the practice of the invention, I make a heat ofsteel by conventional basic open-hearth practice except that I do notadd any. deoxidizer such as ferrosilicon, aluminum or ferrotitanium andmagnesium, either to the bath in the furnace or to the ladle aftertapping the furnace. The carbon content of the heat is brought downprogressively in the furnace to approximately the desired final value,c. g., 0.65%, and ferromanganese is added as required in the furnace orin the ladle. This may be as little as a half pound per ton in one caseand as much as 25 pounds per ton in another. After tapping, the steel isteemed into big-end-down, bottle-top ingot molds, i. e., without hottops, with the addition of about 2.5 pounds per ton of ingot weight, ofa rimming agent composed of 68% globular iron oxide in the form ofdeseamer dust, 17% sodium fluoride, 10% sodium nitrate and 5% granularaluminum. After a predetermined time for rimming, depending on thethickness of case desired, i. e., from 15 seconds to 5 minutes, therimming action is substantially arrested by chemically precaping, viz.,the addition of about 2 oz. per ton of ingot weight, of a suitabledeoxidizer such as aluminum or calcium-silicon alloy, after which themolds are mechanically capped.

One example of steel made in accordance with the invention gave thefollowing analyses:

Table 11 Per- Per- Per- Per- Per- Percent cent cent cent cent cent 0 MnP S Si Al Ladle analysis .67 .22 .015 .024 .02 Recheck analysis ofbillets:

Entire cross section Top of ingot 66 20 015 025 01 006 Bottom of ingot.62 .19 Surface of billet-Top of ingot; 45 20 010 O15 center drillingson 2 x 2 billet-Top of ingot... 95 23 019 069 Ol 006 Other examples ofthe composition of the rimming agent are:

The cross-section of ingots of high-carbon steel produced according tomy invention is characterized by an outer zone or rim area of apredetermined thickness, and this area persists in the billets rolledfrom the ingots. In this area, the carbon and sulphur contents areapproximately two-thirds of the average concentration (ladle analysis)and there is an almost complete freedom from inclusions, giving thesteel exceptionally good drawing properties. The carbon and sulphurcontents at the center of the cross-section are 1.5 or more times theaverage concentration.

Ingots of steel made by my method may be hot-rolled into blooms andbillets as well as conventional low-carbon rimmed steel (i. e., steelcontaining .12% carbon and manganese) and exhibit freedom from pipegiving a higher yield, and also good surface condition.

Segregation in the ingots is well controlled and may easily be keptbelow the limits established for many applications. The case or rim iscleaner and freer of inclusions than normal killed steel and thiscondition persists even at the top center of the ingots. The carbongradient from surface to center gives desirable hardening properties.The steel when rolled into wire rod draws well into fine wire and isalso easy to cold-roll into strip. The absence of silicon and aluminumand freedom from inclusions improve the electrical conducticity, makingthe steel particularly desirable for the production of the high-strenghwire used in communication lines.

Although I have disclosed herein the preferred practice and embodimentof my invention, I intend to cover as well any change or modificationtherein which may be made without departing from the spirit and scope ofthe invention.

I claim:

1. In a method of producing steel, the steps consisting in making a heatof steel containing over .35% carbon, teeming the steel while liquidinto ingot molds and, not substantially later than the early part ofteeming each mold, adding to the steel contained therein from one tofour pounds per ton of the ultimate weight of the ingot,

of a mixture consisting essentially of from 40 to 80% iron oxide, from 5to 20% sodium fluon'de, from 2.5 to 17% granular aluminum and from 5 to35% sodium nitrate, and then when teeming is completed, permittingrimming of the steel to proceed unchecked for a predetermined time.

2. In a method as defined in claim 1, characterized by said mixtureconsisting essentially of about 68% iron oxide, about 17% sodiumfluoride, about sodium nitrate and about 5% aluminum.

3. In a method as defined in claim 1, characterized by chemicallyprecapping the ingot after said predetermined rimming time.

4. In a method as defined in claim 1 characterized by finally cappingthe molds mechanically.

5. An exothermic reaction mixture effective to produce rimming ofhigh-carbon steel, consisting essentially. of from to iron oxide, from 5to 20% sodium fluoride, from 2.5 to 17% granular aluminum and from 5 to35% sodium nitrate.

6. As an article of manufacture, a rolled billet of ductile rimmedopen-hearth steel containing from .35 to 1.10% carbon, having an outercase substantially free from inclusions, the carbon content of said casebeing approximately two-thirds of the average carbon content of thebillet as a whole, and the carbon content adjacent the cente line of thebillet being about 1.5 times said average.

7. As an article of manufacture, a rolled billet of steel adapted to befurther hot-rolled, containing from .35 to 1.10% carbon and having anouter case substantially free from inclusions, the carbon content ofsaid case being approximately two-thirds of the average carbon contentof the billet as a whole and the carbon content of the billet adjacentits longitudinal axis being greater than said average.

References Cited in the file of this patent UNITED STATES PATENTSKlepsch Oct. 7, 1930 OTHER REFERENCES

1. IN A METHOD OF PRODUCING STEEL, THE STEPS CONSISTING IN MAKING A HEATOF STEEL CONTAINING OVER .35% CARBON, TEEMING THE STEEL WHILE LIQUIDINTO INGOT MOLDS AND, NOT SUBSTANTIALLY LATER THAN THE EARLY PART OFTEEMING EACH MOLD, ADDING TO THE STEEL CONTAINED THEREIN FROM ONE TOFOUR POUNDS PER TON OF THE ULTIMATE WEIGHT OF THE INGOT, OF A MIXTURECONSISTING ESSENTIALLY OF FROM 40 TO 80% IRON OXIDE, FROM 5 TO 20%SODIUM FLUORIDE, FROM 2.5 TO 17% GRANULAR ALUMINUM AND FROM 5 TO 35%SODIUM NITRATE, AND THEN WHEN TEENING IS COMPLETED, PERMITTING RIMMINGOF THE STEEL TO PROCEEDING UNCHECKED FOR A PREDETERMINED TIME.